1. Field of the Invention
The present invention relates to a method for treating phenol, particularly to a continuous method for removing hydroxyacetone as well as methylbenzofuran from a crude phenol stream.
2. Description of the Related Art
The process for preparing phenol from cumene is well known. In this process cumene is at first oxidized by air oxygen to cumene hydroperoxide. This process step is typically called oxidation. In the second reaction step, the so-called cleavage, the cumene hydroperoxide is cleaved to phenol and acetone using a strong mineral acid as catalyst, for example sulfuric acid. The product from this second reaction step, the so-called cleavage product, is then fractionated by distillation.
The purity requirements for phenol to be marketed are becoming more and more stringent. Consequently, in order to operate a phenol production plant economically, overall yield and selectivity to the desired end product has to be improved and impurities formed during any of the above-described reaction steps have to be removed as quantitatively as possible with the lowest possible loss of the desired end product, especially phenol and acetone, at low investment and variable costs, especially energy costs. The predominant by-products formed in the oxidation steps are dimethylbenzyl alcohol and acetophenone. Acetophenone leaves the process with the high-boilers from the distillation. Dimethylbenzyl alcohol is dehydrated in the cleavage step to alpha-methylstyrene which partially forms high-boiling dimers and cumylphenols in the acid catalyst cleavage step. The high-boilers are separated from phenol in the distillation step. The unreacted alpha-methylstyrene is separated and hydrogenated in order to form cumene that is recycled into the process. Depending on the market demand, alpha-methylstyrene can also be further purified and sold as value product. Thus, one focus in the prior art is how to operate the oxidation step as well as the cleavage step in order to reduce the formation of these high-boilers which can be considered as direct cumene losses. For example for the cleavage these methods are described in U.S. Pat. No. 4,358,618, U.S. Pat. No. 5,254,751, WO98/27039 and U.S. Pat. No. 6,555,719.
But besides these high-boilers other components are formed in the cleavage, as for example hydroxyacetone, 2-methylbenzofuran and mesityloxide. These so-called micro impurities are not easy to separate from phenol in the distillation. Hydroxyacetone is the most critical component as it is nearly impossible to separate it from phenol by distillation. Hydroxyacetone is typically also the impurity with the highest concentration in the product obtained from the cleavage step. The concentration of hydroxyacetone in the cleavage product may vary between 200 and 3.000 wppm (weight parts per million).
Thus, there are great efforts in the prior art to remove and separate hydroxyacetone from the product obtained from the cleavage step (see for example U.S. Pat. No. 6,066,767, U.S. Pat. No. 6,630,608, U.S. Pat. No. 6,576,798 and U.S. Pat. No. 6,875,898). The disadvantage of all these methods is that high volume flows of cleavage product must be processed. In addition, in U.S. Pat. No. 6,875,898, the high volume flow of cleavage product must be treated with an oxidizing agent that may cause enormous efforts to operate the process safely.
Prior to distillation, the cleavage product is neutralized with a basic aqueous solution such as sodium phenate or caustic soda. The cleavage product which is saturated with water is then worked-up by distillation. A well known method is to separate most of the hydroxyacetone with an aqueous phase which is separated in the first distillation column while a crude phenol together with the high-boilers is taken as the bottom product, as described in U.S. Pat. No. 3,405,038 or in U.S. Pat. No. 6,657,087. In any case the crude phenol which will be further worked up in successive columns will still have concentrations of hydroxyacetone, 2-methylbenzofuran and mesityloxide of some 100 wppm which are not tolerable in pure phenol or even high purity phenol.
DE-AS 1 668 952 discloses a method to remove carbonyl-containing components, like mesityloxide, isomesityloxide, methylisobuylketone, hydroxyacetone and acetophenone, from crude phenol by passing the crude phenol over acidic ion exchange resins at temperatures between 45 and 200° C., preferably between 80 and 150° C.
This reference also discloses the possibility to use two different ion exchange resins whereby—when using such a catalyst combination—different temperatures can be used in order to optimize the efficiency for each catalyst type. This reference is totally silent with respect to the removal of methylbenzofuran nor does this reference disclose that reactors containing the ion exchange resin are operated using a well-defined temperature profile throughout the series of reactors.
As is evidenced by the introductory part of DE-A 199 51 373, the process disclosed in DE-AS 1 668 952 is not suitable to remove low activity carbonyl compounds like methylisobutylketone and methylcyclopentenone and compounds like methylbenzofuran. Consequently, the process described in DE-AS 1 668 952 does not solve the problem of removing hydroxyacetone as well as methylbenzofuran from a crude phenol stream.
In U.S. Pat. No. 5,414,154 a method for purification of phenol is described wherein a phenol stream is contacted with an ion exchange resin at temperatures between 70 and 120° C. It is emphasized in that patent that the effectiveness of the treatment with ion exchange resin increases with increasing temperature taking into account the range of temperature stability for the resin. But it is furthermore evident that methylbenzofuran can only be separated from the crude phenol stream if the initial hydroxyacetone concentration in the crude phenol stream is below 260 wppm and preferably hydroxyacetone is completely removed in order to have hydroxyacetone present in an amount of 10 wppm in order to effectively remove methylbenzofuran when contacting the crude phenol stream with the ion exchange resin.
Consequently, the process described in U.S. Pat. No. 5,414,154 requires an effective removal of hydroxyacetone in upstream units prior to the purification on the ion exchange resin. This is a disadvantage because any removal of hydroxyacetone in upstream units prior to the purification of the ion exchange resin means an extra effort in terms of investment costs and variable costs. Examples of such methods are described in U.S. Pat. No. 6,066,767, U.S. Pat. No. 6,630,608, U.S. Pat. No. 6,576,798 and U.S. Pat. No. 6,875,898.
DE-A 1 995 373, taking into account the disadvantages of DE-A 1 668 952 that 2-methylbenzofuran cannot be effectively separated by the method disclosed therein and the disadvantage of the teaching of U.S. Pat. No. 5,414,154 that methylbenzofuran can only be effectively removed from the crude phenol if the hydroxyacetone content of the crude phenol stream treated with the ion exchange resin is below a certain limit, suggests a two-step process of contacting the crude phenol stream with an ion exchange resin with a thermal separation step, like a distillation step, between the two steps of contacting the crude phenol with an acidic ion exchange resin. This method has the tremendous disadvantage that an additional distillation step is necessary which considerably increases the investment and energy costs of the entire process.
Finally, the teaching of US 2005/0137429 tries to avoid these disadvantages of multi-step purification of crude phenol in order to remove hydroxyacetone and methylbenzofuran by using a one-step process whereby the crude phenol is contacted with an ion exchange resin at low temperatures between 50 and 100° C. Although this one-step process is indeed effective in reducing the hydroxyacetone as well as the methylbenzofuran, this method need relatively high reactor volumes and/or highly activated ion exchange resins because of the lower reaction rate at this relatively low temperatures.
In view of the prior art as discussed above, there is still a need for an effective and economical method for purification of crude phenol to reduce hydroxyacetone as well as methylbenzofuran and other impurities in order to produce high purity phenol.